The present invention relates to a temporary chip attach (TCA) carrier for use in an integrated circuit die testing system; and, more particularly, to a surface metallization structure of the TCA carrier which makes it possible to conduct multiple chip test and burn-in processes in a cost effective manner.
There is a growing need in the microelectronics industry for known-good-dies (KGD""s). These are semiconductor chips (or dies) that have been tested and burned-in, and are known good prior to their sale or being placed on a multi-chip module (MCM).
One common method used today to facilitate the test and burn-in of a new semiconductor chip is to mount the chip on a temporary ceramic carrier, execute the test and burn-in thereof, and then remove the die from the carrier in such a way that neither the quality of the die nor the functionality of solder balls on the die is compromised, wherein the solder balls are attached to the die that join the die to the carrier.
While it is necessary to have the chip securely mounted or electrically connected to the carrier to carry out the test and burn-in, the chip must be easily removable from the carrier without incurring any damage to the chip or the solder balls. In this way, the chips so tested can then be reflowed onto the MCM, or sold to an OEM customer as KGDs so that the tested and/or burned-in chips can be incorporated onto their substrate for final use.
A ceramic carrier used for this purpose is called TCA that stands for temporary chip attach. Certain key aspects that any successful TCA carrier should possess include: (1) proper surface metallization; (2) appropriate electrical contact with all solder balls; (3) ease of chip attachment and detachment; (4) minimal impact to the solder balls for subsequent chip reflow; (5) low cost of TCA carrier manufacture and process; (6) multiple use capability; (7) soft and hard rework capability, to name a few.
Today, there are TCA carriers available whose TCA top surface metal (TSM) contact pads are produced by a thin film process. In general, using the thin film process, a surface metal pad of a reduced area is deposited, through a complex series of steps, on a prepared TSM of a ceramic substrate and located within the circumference of an existing refractory metal via.
An exemplary via and pads of the prior art are illustrated in FIGS. 1A and 1B which respectively show a top and a cross sectional views of a portion of a TCA carrier 12 comprising a refractory metal via 14 and a surface metal pad 16. The reduced area surface metal pad 16 deposited by employing the thin film process is made of a solderable metal such as nickel (Ni). Therefore, the reduced area solderable nickel pad 16 becomes a small island of wettable contacts in a sea of non-wettable refractory metal via 14. These reduced area solderable nickel pads serve to provide a reliable electrical connection with solder balls on a chip (not shown) and also enable the chip to be subsequently cold-sheared off of the TCA carrier 12 after the completion of the test and burn-in thereof, without a significant alteration of the solder balls.
In a more detailed description of the thin film process for the manufacture of a conventional TCA carrier, a standard ceramic substrate, typically an alumina substrate, having about 5 mil vias filled with a refractory metal, e.g., molybdenum (Mo) or tungsten (W), on a TSM layer is sintered. After the sintering, the substrate is coated with polyimide, and then developed and exposed to provide a 1xc3x972 mil opening in the polyimide over each 5 mil via. Although a nickel pad of any general shape may be produced in the thin film process, in this specific example, a rectangular shape has been used for the sake of illustration. Circles, squares, and other shapes may be equally feasible. The substrate is then placed in a Ni/B bath and the 1xc3x972 mil opening is plated with said Ni/B. After the polyimide is stripped away, the newly deposited Ni/B pads are diffused to remove the boron (B) to establish a reliable bond between the nickel pad and the refractory metal via. The substrate is then ready for use as a TCA carrier. The solder ball on the chip wets only the small Ni area on the carrier, creating the electrical connection. The 5 mil via of, e.g., Mo circumscribing the deposited thin film nickel pad is not wetted by the solder ball of a Pb/Sn solder. In this way, the solderable area of the standard 5 mil via is reduced to about 10% of its original area.
While the conventional TCA carrier described above satisfies the key technical aspects, owing to the complex and costly thin film process employed in the manufacture thereof, there has existed a need to develop a low cost TCA carrier.
It is, therefore, a purpose of the present invention to provide a TCA carrier employing a cost-effective and improved surface metallization structure for use in performing multiple chip test and burn-in processes.
The invention relates to a surface metallization scheme wherein a novel and high grit conductive metal paste is used to fill vias in the TSM layer of a TCA carrier to produce fired vias which are comprised of metallic and non-metallic regions. This concept can be applied to standard alumina substrates with refractory metal conductors or to LCGC (low cost glass ceramic) substrates with more noble metal conductors. Depending on the substrate material used, optimal nickel plating conditions are selected to plate the metallic regions of the via with a thin layer of Ni, without bridging them to the non-metallic regions. The resulting surface metallization structure is essentially composed of islands of solderable metallic contacts surrounded by non-metallic and non-wettable regions in the via surface. The metallic contacts can be wetted by Pb/Sn solders of solder balls during chip attach. The non-metallic regions of the composite via remain unwetted, thereby producing a reduced solderable area of the via. This scheme can be combined with the idea of using a smaller via diameter for a further reduction in the solderable area.
In accordance with the present invention, there is provided a ceramic carrier for use in multiple chip test and burn-in, comprising: a substrate; a plurality of vias in the substrate; and a conductive material at least partially filling the vias, the whole top surface of the conductive material being solderable.